There are victims like Emily Moore, a schoolgirl from the country who signed' />

It's not just celebrities who have their reputations trashed online.

There are victims like Emily Moore, a schoolgirl from the country who signed up to a social media site because she just wanted to talk to her friends; and there are powerful figures such as Louis Bacon, the reclusive billionaire, who finds himself hounded into the public gaze by anonymous and vitriolic bloggers.

But it has, perhaps predictably, taken the digital demolition of two famous and glamorous women to bring to the fore the issue of internet defamation, where the good names of individuals can be impugned from - apparently - a place beyond the reach of the libel courts. First Gabby Logan, the television sports presenter, and then Jemima Khan, the writer and campaigner, found their reputations stained with the lava of spite and anger that has erupted online in response to the use of super-injunction gagging orders by the rich and powerful. Neither Logan nor Khan had taken out such an injunction, but both were victims of false claims which implied they were having affairs with married men.

Meanwhile, Kate McCann, who became a publicly known figure only in the most tragic circumstances, but needs to maintain a high-profile in the hope that her missing daughter might one day be found, has spoken of how her personality has been torn to pieces online by people who have never met her.

What was once tittle-tattle for the public bar or the garden fence can be much more damaging in the digital world, where malicious gossip can find an audience of millions - and remain in cyberspace indefinitely.

No one has yet identified a remedy for such hurt. Khan sought to neutralise the slurs on Twitter by appealing to her own 60,000 followers on the micro-blogging site. She warned them that the falsehoods would lead to her children being bullied in school and, as she did so, she alerted many more to the rumours. Logan's response was to issue a statement to the Daily Mail, which publicised the issue on its front page of the more-than two million copies it sold that day.

According to powerful public relations figure Lord Bell, stars that have their reputations trashed online must be careful not to overreact. "I think it was the chief executive of Google who said there were a billion blogs and they have an average readership of one.

"Some of these things that are written online only reach a large audience because they are [then] reported by traditional media," he said. But the gossip directed at Khan had not circulated at the margins of the internet. The accuser on Twitter, who cannot be named for legal reasons, quickly amassed more than 100,000 followers and hundreds of thousands more will have read the slurs.

Publicist Julian Henry, who works for XIX, which has a client list that includes the television show American Idol and the singer Annie Lennox, said there were two approaches victims of online mudslinging could take.

"You put your head down and ignore it or you come out fighting," he said. "The 'lying low' strategy is the more risky, because you are handing control of the agenda to third parties, whether that's newspapers, the Twitterati or Facebook users."

Those that are more proactive need to "come out with all guns blazing and reclaim the agenda", he said. "You have got to be honest, transparent and visible. If you can do that and rubbish the claims, you emerge with a clean slate."

Others, like former bailiff Raymond Bryce (who had child porn posted on his Facebook page by a former friend), managed to pursue their tormenters in the courts.

Those who, like Bacon, do not know the identity of their persecutors, may have difficulty in curtailing their activities. But another leading publicist, Mark Borkowski, said he had received co-operation from Facebook and Twitter when his clients were the subject of abuse.

"Their image rights were being exploited and there were some horrendous things being said on their behalf and people were being inveigled into scams. Facebook and Twitter were really supportive over that, although I might have got lucky."

As for the Twitter-user who made the super-injunction allegations, his actions may have created a precedent in cyber-bullying - or they may have invoked a new level of wrath against its proponents. "Heaven and Earth will try to out this person. The legal world is so embarrassed - they want this person and it's a bigger manhunt than bin Laden," said Borkowski.